[Issue 17541] New: @nogc template deduction problem (codegen different from .mangleof)

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=17541

          Issue ID: 17541
           Summary: @nogc template deduction problem (codegen different
                    from .mangleof)
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: johanengelen at weka.io

I unfortunately only have a complex testcase for this issue (after long
dustmiting sessions and manual reduction), involving three files (one.d, two.d,
three.d).

When only `one.d` is compiled, function `three.TT!(int).insertabcdefg(int)` is
deduced to be `@nogc`. But the deduction happens late, and printing `.mangleof`
does not show it. The symbol in the object file does show `@nogc` in the
mangling.

When `one.d` and `two.d` are compiled simultaneously (with or without
`three.d`), the function is no longer deduced to be `@nogc`.

(The bug was found in Weka's codebase where everything-at-once compilation is
impossible, and where per-package compilation is done. The testcase is heavily
simplified.)

Details:

Only compiling one.d gives:
> dmd one.d -c -of=tmp1.o
_D5three9__T2TTTiZ2TT13insertabcdefgMFiZv
> nm tmp1.o | grep "insertabcdefg"
0000000000000358 S _D5three9__T2TTTiZ2TT13insertabcdefgMFNiiZv

Compiling all three at once gives:
> dmd one.d two.d three.d -c -of=tmp2.o
_D5three9__T2TTTiZ2TT13insertabcdefgMFiZv
> nm tmp2.o | grep "insertabcdefg"
0000000000000460 S _D5three9__T2TTTiZ2TT13insertabcdefgMFiZv




File one.d
```````````````````````````
module one;

import two;
import three;

struct BB
{
    enum MAX_NUM_FIBERS = 4096;

    TWOR!1 t;
    TT!(int) tt;
    auto foo()
    {
        tt.insertabcdefg(1);
    }
}

BB bb;
```````````````````````````


File two.d
```````````````````````````
module two;

import one;

struct ET(bool a)
{
    enum e = BB.MAX_NUM_FIBERS;
}

alias Event = ET!false;

struct TWOR(size_t M)
{
    Event e;

    void open()
    {
        bb.foo();
    }
}
```````````````````````````


File three.d
```````````````````````````
module three;

void aaa() @nogc
{

}

struct TT(T)
{
    void insertabcdefg(T) // @nogc  <-- deduction problem
    {
        pragma(msg, insertabcdefg.mangleof);
        aaa();
    }
}
```````````````````````````

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